A hitherto-used rotation detecting device comprises a permanent magnet and a pair of yokes that are rotatable relative to each other, a pair of magnetism collecting rings that induce magnetic fluxes produced between the paired yokes due to the relative rotation between the permanent magnet and the paired yokes, and a Hall IC sensor that detects the magnetic fluxes between the magnetism collecting rings. The paired yokes and the paired magnetism collecting rings that constitute a detecting member are constructed of a soft magnetic material such as Permalloy (permeability alloy) or the like and integrally molded with a resin holder through an insert molding technique, and the yokes and magnetism collecting rings are connected to input/output shafts or a housing through the holder. One example of the above-mentioned technology is described in Patent Document 1.